Owing to excellent combinations of strength and weight, composite materials are being increasingly used to replace aluminum in aircraft structures. Although this affords significantly increased fuel efficiency and/or greater payload capacity, aircraft structures unfortunately become more vulnerable to lightning damage. This increased vulnerability is rooted in the inferior electrical conductivity of composites, such as those based on carbon fiber reinforced materials, relative to that of aluminum metal. Naturally, the less conductive a material is the more energy that it will absorb owing resistive heating mechanisms. It has been reported that carbon fiber composites can absorb nearly 2,000 times the amount of energy from lightning strikes as compared to the same mass of aluminum. The increased absorbed energy leads to increased “direct” and “indirect” effects.
Direct effects are associated with physical or “direct” damage to load bearing structures, with the worst types of damage being severe punctures through composites laminates. “Indirect” effects are associated with electrical surges caused by the lightning's massive electromagnetic field. These surges can disrupt avionics and in turn compromise the pilot's ability to control the aircraft. Indirect effects are even more of concern lately as aircraft controls are increasingly moving towards fly-by-wire systems. It is for this reason why massive amounts of electromagnetic interference (EMI) shielding materials in the form of boxes, gaskets, metal foils and meshes, adhesives, metal sheathing, etc. are used to shield electrical components, wiring, and connections.
In order to protect composites against the aforementioned effects, aircraft designers seek to keep the strong electrical currents on the outer surface of the aircraft by integrating highly, conductive skins in the composite structure. Numerous attempts to produce such lightning strike protection (LSP) skins have been made and/or proposed, each with varying degrees of success. For example, metal wire meshes and expanded metal foils (EMF) based on metals such as copper, aluminum, or bronze have been embedded in a surfacing (or adhesive) films and co-cured with underlying composite prepregs. Alternatively, individual wires have been interwoven with carbon fibers to produce hybrid prepregs. Similarly, metal deposition techniques have been employed to coat carbon-fibers or other reinforcing fibers in their raw or woven forms. In addition to metalized fibers, flame spray is another LSP approached used, which involves depositing molten metal, typically aluminum, onto substrates.
Recently, conductive films and adhesives have been promoted which provide lightning strike protection while reducing weight, cost, and facilitating easier installation and repair. These materials are discussed in U.S. Patent Application Publication No. 2011/0014356.
All of the aforementioned LSP systems attempt to maximize protection of the substrate while minimizing weight and cost.